This invention relates to the deposition of thin films and is more concerned with a reactive sputtering process wherein atoms of a target material are freed from a conductive target and are reacted with a reactive gas to form a coating which is deposited on a substrate. This process can be employed, for example, in creating dielectric insulating layers on electrical parts, or wear-resistant layers on mechanical parts.
The invention is more specifically directed to a dc sputtering process in which the dielectric coating material that becomes lodged on the conductive target is removed, thus avoiding a major cause of arcing.
Sputtering is a vacuum deposition process in which a sputtering target is bombarded with ions, typically an ionized noble gas, and the atoms of the target material are mechanically freed by momentum transfer. The target material then coats a nearby substrate.
In a reactive sputtering process a reactive gas is introduced into the deposition chamber, and the freed target material reacts with the reactive gas to form a coating material. For example, the target material can be aluminum, and oxygen can be introduced as the reactive gas to produce a coating of aluminum oxide. A carbonaceous gas, e.g. acetylene can be used as the reactive gas to produce carbide coatings such as SiC, WC, etc., and ammonia can be introduced to produce a nitride coating such as TiN. In any event, the conductive target atoms and the reactive gas react in plasma in the chamber to produce the compound that serves as a coating. In a typical example, aluminum atoms freed from an aluminum target enter plasma of argon and oxygen to produce a deposition of aluminum oxide.
DC sputtering is a random process and the insulating coating material is deposited on all available surfaces. This means that not only does the insulating material coat the article in question, but it also coats other surfaces in the chamber as well, including the target. Thus, in a reactive sputtering process for depositing aluminum oxide, molecules of Al.sub.2 O.sub.3 land on the surface of the aluminum target. This deposition of an insulator on the target causes severe problems, including a reduction of sputtering rate and a propensity to induce arcing.
Contamination of the target can also result, even in conventional dc sputtering, due to atmospheric gases, water droplets, inclusions, and other contaminants. Each of these can be a source for arcing, and their presence will also reduce the deposition rate over time because of a reduced active sputtering area on the target. Accordingly, these problems necessitate a frequent cleaning of the target surface.
This problem has been known to exist for some time, but its causes have not been completely appreciated. Procedures to deal with these problems, such as arc control in reactive sputtering have not been completely satisfactory.
A standard approach involves sensing the presence of arcing, and then interrupting current flow. This will control the arcing, but does nothing about the insulating coating that continues to cover the target.
One early attempt to deal with arcing in a blind fashion involves periodically interrupting the current flow between the dc power supply and the plasma generator in which the sputtering occurs. Here, shutting off the dc power serves to extinguish early arcing. This means that unipolar pulses of power at a fixed duty cycle are fed to the target. This does have the beneficial effect of permitting charge to build up only partially across the dielectric deposition on the target, so that arcing is less likely to occur, and also can lead to a small amount of resputtering of the deposition. However, this system while slowing down the rate of insulating deposit on the target, does not reverse the deposition.
Another system previously proposed is called a low energy small package arc repression circuit. In that system, an electronic switch cycles at a rate of about 2 KHz to cut out the current to the target. This in effect reverses the voltage on the target to several volts positive, and draws some electrons from the plasma to the front surface of the insulative deposition. This neutralizes the anions on the front surface of the deposition, to discharge the voltage buildup across the layer, thereby greatly reducing the occurrence of dielectric breakdown and arcing. Also, discharge of the front surface of the insulative layer lowers the surface potential to approximately that of the target. Discharging of the dielectric deposition also permits the argon ions in the plasma to collide with the insulating dielectric material. This does result in some resputtering of the molecules of the deposited material, thus slowing the rate of deposit on the target.
However, this approach does not resputter the molecules of the deposited compound as effectively as the atoms of the target material, and so this approach has not been totally effective in removing deposits from the target during reactive sputtering processes.
Different materials require different voltages to be applied to the targets to effect sputtering. For example, because a gold atom is a much heavier atom than an aluminum atom, it requires a much more energetic ion to free it from the target. Typically, in a process that employs an aluminum target, the applied voltage needed is about -450 volts, while in a similar process that employs a gold target the applied voltage has to be about -700 volts.
Considering that an aluminum oxide (Al.sub.2 O.sub.3) molecule is significantly heavier than an aluminum atom, one can understand that a higher potential would be required to energize the argon ion enough to resputter the coating. This is, of course, true for other materials as well.
Another approach to solve this problem involves a pair of sputtering targets, with one serving as cathode and the other as the anode. The applied electrical voltage is periodically reversed so that the sputtering occurs first from the one target and then from the other. This process also reverses the charge on the deposited insulating material as well, which reduces the possibility of arcing and also resputters some of the insulating material on the targets. However, this arrangement, requiring plural targets, can be cumbersome and expensive to employ.
These previous solutions, which employ unipolar pulsing or alternately cycled targets, have been effective in reducing voltage stress on insulating films redeposited on the targets, but have not been effective in removing the redeposit or in preventing it. None of these approaches sputters off the insulator from the beginning before it has a chance to accumulate, and none of these techniques has been effective in eliminating or halting the redeposition of insulating film on the target.